Many devices use steam to treat garments and other objects to remove wrinkles, for cleaning or for other purposes. For example, a steam iron discharges steam from a soleplate onto a garment to help remove wrinkles. In another example, a steam cleaner may comprise a hose with a steam applicator that a user moves to direct steam onto fabrics, such as curtains or upholstery. Typically these devices comprise a steam generator that heats and evaporates water to produce the required steam. Many other applications also require steam, such as a steamer for heating food or a steam cabinet for sterilizing objects. Such devices typically go through periods of use followed periods of non-operation and this causes regular heating and then cooling of the device.
There are two common ways to evaporate water within such devices to produce steam: firstly, water can be pooled and heated to beyond boiling point to produce steam; secondly, water can be sprayed or dropped onto a heated evaporation surface which evaporates the water droplets as the water contacts the evaporation surface and creates a film which is of water on the evaporation surface. In both cases, evaporation of the water results in scale accumulating on evaporation surfaces where the evaporation occurs. Scale forms when water is evaporated and impurities and other substances which were dissolved in the water are left behind and form solid compounds. All non-ionized water will have such impurities, but scale is particularly common in areas where the mains water supply is hard water, i.e. it contains a relatively high level of impurities such as calcium and magnesium.
Presently, scale must be removed from devices to maintain performance and reliability. Scale accumulation on evaporation surfaces within the device will detrimentally affect the heating performance of the device because the scale will act to insulate the heating elements and may also block passageways. In many cases scale will accumulate on the heating element as this is where the evaporation occurs. The scale may be retained on the heating element or evaporation surface or it may flake off and be loose within the device.
Moreover, as water is heated it may react with any accumulated scale and this can result in a foam substance being produced and the heated water and steam may also carry impurities such as small bits of scale. This foam and/or impurities that may be carried by the steam can mark and stain any garment or other material which is being treated as well as cause blockages in other parts of the device.
Presently, scale must be removed by using a cleaning agent, such as a weak acid, or by physically scraping the scale off of the evaporation surfaces. Alternatively, water can be treated before being placed in the device to remove impurities and other dissolved substances and thereby reduce or eliminate the problems of scale. However, all of these methods involve effort and expense and are only partly effective. Scale greatly reduces the lifetime and performance of steam generating devices.
It is known from U.S. Pat. No. 6,167,643 to provide a steam iron in which the flow of water to the evaporation chamber is controlled to ensure that all the water in the evaporation chamber is evaporated to prevent the occurrence of water dripping out of the steam distribution holes when the temperature in the evaporation chamber is insufficient to evaporate all the water admitted into the chamber. In U.S. Pat. No. 6,167,643, scale is deposited directly within the evaporation chamber.